Now, with the NLO result sufficiently stable, the natural next step is to compare the predictions with the data, that is, at the level of the cross section.
In Fig. 3 (left panel), we show predictions using three sets of global PDF fits [1]}, [2]}, [3]}, evaluated at \(Q_0 = \mu _F = \mu _f = \mu _R = m_c\) . The results agree favourably in the HERA regime, where the global partons are better constrained, whilst in the LHCb regime we observe huge differences between the various global PDFsExclusive \(J/\psi \)  photoproduction was also studied within the \(k_t\)  factorisation framework, see [4]}. Here, a DLL modified input gluon ansatz mimicking the resummation of double logarithms through one step of DGLAP evolution was used and fitted to the experimental data. In this approach, however, only a subset of the full NLO corrections are incorporated. As the tamed collinear factorisation result includes all the effects within \(k_t\)  factorisation (to NLO accuracy) but with the full NLO contribution realised, it is in this sense an improved theoretical prediction. Moreover, the gluon extracted in this alternative approach is not readily comparable to the \(\overline{\text{MS}}\)  partons of the global analyses, without a non-trivial scheme conversion. Hence, we do not compare at a quantitative level here.. This, together with the right panel in which we propagate the individual global fit uncertainties to the cross section, shown as a shaded band for MMHT14 and boundary curves for NNPDF3.0, demonstrates the utility of the exclusive \(J/\psi \)  data and supports the claim that their inclusion into
the global PDF analyses would provide a serious constraint for the low \(x\)  and low \(Q\)  behaviour of the gluon PDF.
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